The present invention relates to an aircraft rotor with a constant-velocity transmission. The invention preferably applies to tilt-rotor aircraft or convertiplanes, but may also be used on conventional helicopters, e.g. a tail rotor.
To combine the speed, range and performance of fixed-wing turbo-propeller craft and the hovering and vertical takeoff and landing capability of a helicopter, convertible aircraft or so-called convertiplanes have been devised, which have the same external structure as a turbo-propeller airplane, but are equipped with rotors which can be tilted upwards to provide the lift of a helicopter, and forwards to provide the thrust of an airplane, so that the craft can take off and land in confined spaces like a helicopter, and cruise at high speed and altitude like an airplane. The advantages of such a combination, with respect to a conventional helicopter, are: almost twice the cruising speed (500 as opposed to 300 km/h); substantially twice the distance and time range for a given load and fuel capacity, thus increasing effective utilization of the craft; and over twice the cruising altitude, so that, for most of its flying time, the craft is unaffected by weather conditions (cloud, turbulence). As compared with a conventional airplane, on the other hand, the craft can hover and take off and land in confined spaces.
One of the main dynamic problems of helicopter rotors, and which is further compounded in the case of convertiplanes on account of the anomalous operating conditions of these during conversion, is controlling the vibratory forces acting on the rotor. These are mainly due to the gyroscopic effects produced jointly by tilting the rotor axis with respect to that of the transmission shaft, and variations in angular velocity when the rotor transmission fails to ensure perfectly constant-velocity transmission from the main transmission shaft to the rotor hub.
Various types of aircraft rotor transmissions are known. In one solution, the hub is supported on a spherical bearing to rotate about a pole, and is connected to a drive member, integral with the main shaft, by a transmission member in the form of a substantially toroidal, flexible member and made of composite material. Though relatively straightforward in design, the dynamic problems described above result, when the transmission member is deformed under load, in bobbing of the hub, which in turn results in additional inertial and aerodynamic forces on the blades.
In other known solutions, the hub is connected to the drive member, integral with the transmission shaft, in articulated manner by means of a number of compasses or tangential rods. Such solutions normally pose more or less serious problems of vibration, depending on the kinematic arrangement adopted, and are complicated, subject to wear, are unreliable, and adapt poorly to different design specifications.
It is an object of the present invention to provide an aircraft rotor designed to eliminate the aforementioned drawbacks typically associate with known rotors.
According to the present invention, there is provided an aircraft rotor comprising a drive shaft, a number of blades extending substantially radially with respect to the drive shaft, and a transmission connecting said blades to said drive shaft; the transmission comprising a hub supported adjustably about a fixed pole, articulating means connecting each of said blades to said hub, and torque-transmitting means connecting said drive shaft to said hub; characterized in that said hub is symmetrical with respect to a plane of its own through said pole; and in that said torque-transmitting means comprise a drive member rotationally integral with said drive shaft, a driven member connected rigidly to said hub, and a number of torque transmitters, each connected to a first attachment carried by the drive member, and to a second attachment carried by the driven member, by means of a first hinge and a second hinge respectively; said first and second hinges having axes lying respectively in a first and a second plane on opposite sides of and symmetrical with respect to a bisecting plane coincident, in stationary rest conditions, with said plane of said hub.